A substantial amount of radioactive waste has been generated over the last several decades. Such waste is generated, for example, as a by-product of nuclear weapons construction and destruction, or as a by-product of nuclear reactor power plants in the form of a spent nuclear fuel. Radioactive waste poses a threat to humans, and the severity of the threat varies as a function of the type and amount of radioactive atoms present in the waste. Accordingly, it is desirable to develop methods for identifying and quantitating radioactive atoms present in a nuclear waste.
Radioactive atoms present in nuclear waste are generally atoms that undergo fission decay processes. In other words, the atoms "split" and thereby convert from one element to another. Decay particles, such as .gamma.-rays, beta-rays, alpha-rays, and neutrons can be generated during the splitting. Numerous decay particles can be generated by a single fission event. For instance, a single fission event for .sup.242 Pu can emit multiple .gamma.-rays and multiple neutrons. The .gamma.-rays travel at the speed of light, and the neutrons travel slower than the speed of light. The neutrons can be so-called "fast neutrons" and so-called "thermal neutrons". The fast neutrons have energy greater than or equal to 40 keV, and the thermal neutrons have energy less than 40 keV (actually, about 0.25 eV).
Detection systems have been developed for detecting decay particles. A common system couples a liquid scintillator solution with a phototube. The liquid scintillator comprises a mixture of volatile hydrocarbons (such as, for example, xylene, naphthalene, and/or anthracene) that have been purged of oxygen and other oxidizing radicals. The liquid scintillator typically has a high ratio of hydrogen to carbon, which can make it particularly effective for detecting neutrons. Specifically, neutrons can scatter off the protons (hydrogen atoms) to thereby transfer energy to the protons. As the protons recoil from such scatter, they can emit light. Such light can then impact the photo tubes to be converted to an electrical signal. Frequently, only a very small amount of light is produced. Accordingly, the photo tubes frequently comprise photomultiplier tubes which are configured to amplify a light signal as well as to convert the light signal to an electrical signal. The liquid scintillator can be varied depending upon whether it is desired to detect thermal neutrons or fast neutrons. Specifically, some liquid scintillators are known to detect fast neutrons much better (i.e, at higher efficiency) than thermal neutrons.
The liquid scintillator/phototube detector can also detect .gamma.-rays. Specifically, .gamma.-rays can interact with atomic electrons of atoms in the liquid scintillator via a so-called Compton scattering process to create light.
Exemplary electrical signals produced by a liquid scintillator/phototube detector in response y-ray and neutron stimulation of the detector are shown in a FIG. 1 graph. A curve corresponding to an electrical signal obtained from a .gamma.-ray is illustrated with a solid line, and a curve corresponding to an electrical signal obtained from a neutron is illustrated with a dashed line. The FIG. 1 graph compares an output current from a phototube to time (as measured in nanoseconds). The magnitude of the pulses and the time duration are not relevant for the present discussion, simply the relative differences in the .gamma.-ray curve versus the neutron curve are important. The pulses develop in time from left to right. Each of the curves has a leftmost (leading edge) region "A" wherein an absolute value of current is increasing, and a rightmost (tailing edge) region "C" wherein an absolute value of current is decreasing. Also, the curves have a peak intensity "B" wherein an absolute value of current is maximized.
The greatest difference between the neutron curve and the .gamma.-ray curve occurs in region "C", and the least difference occurs in region "A". Methods have been developed for utilizing the differences in region "C" for distinguishing neutron curves from .gamma.-ray curves. Such methods typically integrate a curve, such as, for example, either the .gamma.-ray curve or the neutron curve shown in FIG. 1 to determine a total intensity underlying the curve. The methods typically also involve separately integrating the intensity under a region "C" of the curve to obtain a final intensity of the curve. The final intensity is then ratioed to the total intensity to obtain a value. The value is compared to standard values obtained from both .gamma.-ray curves and neutron curves to ascertain if the curve in question was generated by a .gamma.-ray interaction with a detector, or a neutron interaction with a detector. The methods thus effectively comprise comparing a final intensity to a sum of the final intensity plus an initial intensity, wherein the initial intensity is defined as the intensity underlying region "A" of a curve.
Methods of utilizing curve pulse shape for distinguishing signals due to neutron stimulation of a detector from signals due to .gamma.-ray stimulation are generally referred to as Pulse Shape Discrimination (PSD) systems. The .gamma.-ray curve and neutron curve shown in FIG. 1 are idealized. Frequently, the curves are not as readily distinguishable as those shown. The PSD values will accordingly be very close for .gamma.-ray curves and neutron curves, and the difference between .gamma.-ray curves and neutron curves will essentially be represented as a small difference in very large PSD values. It is therefore desirable to develop alternative PSD systems wherein differences between neutron curves and .gamma.-ray curves are enhanced. Another complication in characterizing decay particles generated by nuclear waste is in distinguishing such decay particles from background radiation. It is known that background radiation typically occurs as isolated random events, whereas radiation emitted from a waste material can occur in bursts of multiple particles. The bursts result from multiple particles being generated by single fission events. The multiple particles emitted by a single fission event of a radioactive material are referred to coincidence particles. Coincidence can be used to distinguish background radiation from radiation generated by a radioactive sample. For instance, circuitry can be configured to "turn on" a signal acquisition system in response to a detected coincidence event. Exemplary circuitry utilizes multiple detectors to monitor a sample. The detectors are linked to a pair of processing systems. The first processing system is configured to determine coincidence of signals received from the detectors. Once coincidence is established, the first processing system opens the second processing system. Further detection events are then processed by the second processing system which is configured to store and process electrical pulses obtained from the detectors. The first and second processing systems are opened for predetermined lengths of time. Such predetermined lengths of time can vary depending on the stringency with which background radiation is to be suppressed. Coincidence is typically established by signals occurring within 10 microseconds of one another. Once coincidence is established, the second gate is typically opened for a period of at least about 100 microseconds.
An effectiveness of a coincidence method for suppressing background in a given length of time can be limited by a speed that detectors respond to radiation and the time required by associated circuitry for processing signals received from the detectors. If a detector requires microseconds to convert light to an electrical pulse and then output the pulse to a processor, then the maximum count rate at which the detector is usable is only a few hundred thousand counts per second. For example, a detector that requires ten microseconds to process one count has a maximum count rate of 100,000 counts per second. Currently available systems typically have a count rates on the order of about 200,000 counts per second. Such limits the maximum speed with which a system can acquire information from a radioactive sample. It is desirable to develop faster methods of acquiring information from radioactive samples.
Another problem that can occur in monitoring radiation emitted from radioactive samples is that the samples are frequently encapsulated in materials designed specifically to limit an amount of radiation penetrating the materials. Such materials are provided for safety purposes, to limit a risk of exposure of persons near the radioactive waste to dangerous levels of radiation. Frequently, it is required to destroy the retaining materials prior to analysis of a sample in order to have enough radioactive particles impact detectors proximate the sample to obtain information about the sample. It is desirable to develop alternative methods of obtaining information about radioactive samples which can be performed non-destructively relative to retaining materials encapsulating the samples.
Another problem confronted during monitoring of radiation emitted by radioactive samples is in obtaining information about .gamma.-ray emissions. Specifically, .gamma.-rays do not interact well with liquid scintillating materials. Accordingly, most .gamma.-rays simply penetrate the materials without being detected. Improved detectors have been developed for detecting .gamma.-rays. Such detectors are so-called solid state detectors. The detectors utilize crystals of, for example, germanium to convert energy from .gamma.-rays directly to electronic charges. Typically, the detectors will comprise a single crystal of germanium. Such detectors are highly efficient for detecting .gamma.-rays. A disadvantage of the detectors is that they generally will not detect neutrons. It is desirable to develop detector systems which can efficiently detect both neutrons and .gamma.-rays.